1. Field of the Invention
The present invention relates to a magnetic recording medium having improved durability.
2. Description of the Related Art
A magnetic recording medium is usually produced by coating a magnetic paint comprising magnetic powder, a binder, an organic solvent and optionally other additives on a substrate such as a polyester film and drying the coated magnetic paint to form a magnetic layer. The magnetic recording medium is required to have not only good electromagnetic conversion properties, such as high sensitivity and a high S/N ratio, but also good durability. Recently, to improve the performances of the magnetic recording medium, magnetic metal powder which is suitable for recording in a shorter wavelength range is used together with a binder component, such as a polyurethane resin, which contains an aliphatic ester as a polyester component and a vinyl chloride/vinyl acetate copolymer.
When the magnetic metal powder is used together with the binder component, such as the above polyurethane resin and the vinyl chloride/vinyl acetate copolymer, the S/N ratio is improved, although film strength of the magnetic layer deteriorates so that durability, such as still durability of a video tape, is decreased or clogging or contamination of a magnetic head tend to occur.
The present inventors have investigated the causes for the clogging and contamination of a magnetic head, and found that, due to friction between the magnetic recording medium and the magnetic head during traveling, the surface temperature of the magnetic layer rises so that the binder component, such as the polyurethane resin and the vinyl chloride/vinyl acetate resin, is softened whereby the force for retaining the magnetic powder deteriorates, and dropped powder particles cause clogging and contamination of the magnetic head. Further, it was found that, as the glass transition temperature of the binder resin increases, the film strength of the magnetic layer increases, so that clogging and contamination of the magnetic head are suppressed. Further investigation revealed that the glass transition temperature of the binder resin depends on the glass transition temperature of the polyurethane resin present, and when an aromatic ester is used as a polyester component of the polyurethane resin, the clogging and contamination of the magnetic head are prevented, although the repeated traveling durability of the magnetic recording medium is decreased.